1. Field of the Invention
The present invention pertains to a method and apparatus for measuring the electrical resistivity of a subterranean formation through a metal well casing or drillstem utilizing an amplifier circuit which is capable of accurately measuring relatively small electrical voltages.
2. Background
Various techniques have been contemplated for measuring the resistivity of a geologic formation as a means of determining formation physical properties, locating hydrocarbon reserves and related operations. One problem associated with making electrical signal measurements for determining formation resistivity or conductivity pertains to the presence of metal well casing or metal drill pipe, both of which interfere with determining the value of resistance or conductivity of the formation itself.
Some prior art efforts to measure geologic formation resistivity by measuring the current flow through a metal well casing or drillstem have assumed that the resistance of the casing or metal drill pipe is constant. Further work in this general area has revealed that not only the dimensional differences in metal well casing due to manufacturing tolerances, corrosion and other environmental factors affect the resistance of the casing but also the chemical composition of the metal itself causes significant variations in resistivity of casing or drill pipe.
Other problems associated with prior art efforts to measure the resistivity of a geologic formation pertain to the signal level which can be sensed by measuring instrumentation at shallow as well as deep depths of measurement in a wellbore, and also the electrical interference created by certain components of the measurement system, including circuit elements, conductors and connectors. Accordingly, there has been a longfelt need to develop improved formation resistivity measurement techniques and apparatus in order to make this basic process viable not only for exploration operations concerning hydrocarbons and mineral values, but also to carry out improved operations known generally in the art as "well logging."
The present invention overcomes some of the deficiencies of prior art techniques and apparatus and provides a superior method and system for making geologic formation resistivity measurements through metal casing and/or metal drill pipe, for example.